1. Field of the Invention
The present invention is related to the fields of molecular biology, developmental biology, physiology, neurobiology, endocrinology and medicine.
2. Description of Related Art
Parathyroid hormone (PTH) is a major regulator of calcium homeostasis whose principal target cells occur in bone and kidney. Regulation of calcium concentration is necessary for the normal function of the gastrointestinal, skeletal, neurologic, neuromuscular, and cardiovascular systems. PTH synthesis and release are controlled principally by the serum calcium level; a low level stimulates and a high level suppresses both hormone synthesis and release. PTH, in turn, maintains the serum calcium level by directly or indirectly promoting calcium entry into the blood at three sites of calcium exchange: gut, bone, and kidney. PTH contributes to net gastrointestinal absorption of calcium by favoring the renal synthesis of the active form of vitamin D. PTH promotes calcium resorption from bone indirectly by stimulating differentiation of the bone-resorbing cells, osteoclasts. It also mediates at least three main effects on the kidney: stimulation of tubular calcium reabsorption, enhancement of phosphate clearance, and promotion of an increase in the enzyme that completes synthesis of the active form of vitamin D. PTH exerts these effects primarily through receptor-mediated activation of adenylate cyclase and phospholipase C.
Disruption of calcium homeostasis may produce many clinical disorders (e.g., severe bone disease, anemia, renal impairment, ulcers, myopathy, and neuropathy) and usually results from conditions that produce an alteration in the level of parathyroid hormone. Hypercalcemia is a condition that is characterized by an elevation in the serum calcium level. It is often associated with primary hyperparathyroidism in which an excess of PTH production occurs as a result of a lesion (e.g., adenoma, hyperplasia, or carcinoma) of the parathyroid glands. Another type of hypercalcemia, humoral hypercalcemia of malignancy (HHM) is the most common paraneoplastic syndrome. It appears to result in most instances from the production by tumors (e.g., squamous, renal, ovarian, or bladder carcinomas) of a class of protein hormone which shares amino acid homology with PTH. These PTH-related proteins (PTHrP) appear to mimic certain of the renal and skeletal actions of PTH and are believed to interact with the PTH receptor in these tissues. PTHrP is normally found at low levels in many tissues, including keratinocytes, brain, pituitary, parathyroid, adrenal cortex, medulla, fetal liver, osteoblast-like cells, and lactating mammary tissues. In many HHM malignancies, PTHrP is found in the circulatory system at high levels, thereby producing the elevated calcium levels associated with HHM.
The pharmacological profiles of PTH and PTHrP are nearly identical in most in vitro assay systems, and elevated blood levels of PTH (i.e., primary hyperparathyroidism) or PTHrP (i.e., HHM) have comparable effects on mineral ion homeostasis (Broadus, A. E. & Stewart, A. F., “Parathyroid hormone-related protein: Structure, processing and physiological actions,” in Basic and Clinical Concepts, Bilzikian, J. P. et al., eds., Raven Press, New York (1994), pp. 259–294; Kronenberg, H. M. et al., “Parathyroid hormone: Biosynthesis, secretion, chemistry and action,” in Handbook of Experimental Pharmacology, Mundy, G. R. & Martin, T. J., eds., Springer-Verlag, Heidelberg (1993), pp. 185–201). The similarities in the biological activities of the two ligands can be explained by their interaction with a common receptor, the PTH/PTHrP receptor, which is expressed abundantly in bone and kidney (Urena, P. et al., Endocrinology 134:451–456 (1994)).
Native human parathyroid hormone is an unmodified polypeptide of 84 amino acids. It is secreted from the parathyroid glands in response to low blood calcium levels and acts on osteoblast (bone-building cells) in bone, and on tubular epithelial cells of kidney. The hormone interacts with a cell surface receptor molecule, called the PTH-1 receptor or PTH/PTHrP receptor, which is expressed by both osteoblast and renal tubular cells. PTHrP, the major cause of the humoral hypercalcemia of malignancy, also has normal functions that include roles in development. PTHrP has 141 amino acids, though variants also occur that result from alternative gene splicing mechanisms. PTHrP plays a key role in the formation of the skeleton through a process that also involves binding to the PTH-1 receptor (Karaplis, A. C., et al., Genes and Dev. 8:277–289 (1994) and Lanske, B., et al., Science 273:663–666 (1996)).
The PTH-1 receptor is homologous in primary structure to a number of other receptors that bind peptide hormones, such as secretin (Ishihara, T. et al., EMBO J. 10:1635–1641 (1991)), calcitonin (Lin, H. Y. et al., Science 254:1022–1024 (1991)) and glucagon (Jelinek, L. J. et al., Science 259:1614–1616 (1993)); together these receptors form a distinct family called receptor family B (Kolakowski, L. F., Receptors and Channels 2:1–7 (1994)). Within this family, the PTH-1 receptor is unique, in that it binds two peptide ligands and thereby regulates two separate biological processes. A recently identified PTH receptor subtype, called the PTH-2 receptor, binds PTH but not PTHrP (Usdin, T., et al., J. Biol. Chem. 270:15455–15458 (1995)). This observation implied that structural differences in the PTH and PTHrP ligands determined selectivity for interaction with the PTH-2 receptor. The PTH-2 receptor has been detected by RNA methods in the brain, pancreas and vasculature, however, its biological function has not been determined (Usdin, T., et al., J. Biol. Chem. 270:15455–15458 (1995)). It is hypothesized that the family B receptors use a common molecular mechanism to engage their own cognate peptide hormone (Bergwitz, C., et al., J. Biol. Chem. 271:26469–26472 (1996)).
The binding of either radiolabeled PTH(1–34) or PTHrP(1–36) to the PTH-1 receptor is competitively inhibited by either unlabeled ligand (Jüppner, H. et al., J. Biol. Chem. 263:8557–8560 (1988); Nissenson, R. A. et al., J. Biol. Chem. 263:12866–12871 (1988)). Thus, the recognition sites for the two ligands in the PTH-1 receptor probably overlap. In both PTH and PTHrP, the 15-34 region contains the principal determinants for binding to the PTH-1 receptor. Although these regions show only minimal sequence homology (only 3 amino acid identities), each 15–34 peptide can block the binding of either PTH(1–34) or PTHrP(1–34) to the PTH-1 receptor (Nussbaum, S. R. et al., J. Biol. Chem. 255:10183–10187 (1980); Caulfield, M. P. et al., Endocrinology 127:83–87 (1990); Abou-Samra, A.-B. et al., Endocrinology 125:2215–2217 (1989)). Further, the amino terminal portion of each ligand is required for bioactivity, and these probably interact with the PTH-1 receptor in similar ways, since 8 of 13 of these residues are identical in PTH and PTHrP.
Both PTH and PTHrP bind to the PTH-1 receptor with affinity in the nM range; the ligand-occupied receptor transmits a “signal” across the cell membrane to intracellular effector enzymes through a mechanism that involves intermediary heterotrimeric GTP-binding proteins (G proteins). The primary intracellular effector enzyme activated by the PTH-1 receptor in response to PTH or PTHrP is adenylyl cyclase (AC). Thus, PTH induces a robust increase in the “second messenger” molecule, cyclic adenosine monophosphate (cAMP) which goes on to regulate the poorly characterized “downstream” cellular processes involved in bone-remodeling (both bone formation and bone resorption processes). In certain cell-based assay systems, PTH can stimulate effector enzymes other than AC, including phospholipase C (PLC), which results in production of inositol triphosphate (IP3), diacylglycerol (DAG) and intracellular calcium (iCa2+). The roles of these non-cAMP second messenger molecules in bone metabolism are presently unknown.
Osteoporosis is a potentially crippling skeletal disease observed in a substantial portion of the senior adult population, in pregnant women and even in juveniles. The disease is marked by diminished bone mass, decreased bone mineral density (BMD), decreased bone strength and an increased risk of bone fracture. At present, there is no effective cure for osteoporosis, though estrogen, calcitonin and the bisphosphonates, etidronate and alendronate are used to treat the disease with varying levels of success through their action to decrease bone resorption. Since parathyroid hormone regulates blood calcium and the phosphate levels, and has potent anabolic (bone-forming) effects on the skeleton, in animals (Shen, V., et al., Calcif. Tissue Int. 50:214–220 (1992); Whitefild, J. F., et al., Calcif. Tissue Int. 56:227–231 (1995) and Whitfield, J. F., et al., Calcif. Tissue Int. 60:26–29 (1997)) and humans (Slovik, D. M., et al., J. Bone Miner. Res. 1:377–381 (1986); Dempster, D. W., et al., Endocr. Rev. 14:690–709 (1993) and Dempster, D. W., et al., Endocr. Rev. 15:261 (1994)) when administered intermittently, PTH, or PTH derivatives, are prime candidates for new and effective therapies for osteoporosis.
Truncated PTH derivatives such as PTH(1–34) and PTH(1–31) are active in most assay systems and promote bone-formation (Whitefild, J. F., et al., Calcif. Tissue Int. 56:227–231 (1995); Whitfield, J. F., et al., Calcif. Tissue Int. 60:26–29 (1997); Slovik, D. M., et al., J. Bone Miner. Res. 1:377–381 (1986); Tregear, G. W., et al., Endocrinology 93:1349–1353 (1973); Rixon, R. H., et al., J. Bone Miner. Res. 9:1179–1189 (1994); Whitfield, J. F. and Morley, P., Trends Pharmacol. Sci. 16:372–386 (1995) and Whitfield, J. F., et al., Calcif. Tissue Int. 58:81–87 (1996)). But these peptides are still too large for efficient non-parenteral delivery and low cost. The discovery of an even smaller “minimized” version of PTH or PTHrP would be an important advance in the effort to develop new treatments for osteoporosis.
PTH and PTHrP derivatives that have amino acid substitutions or deletions in the 1–14 region usually exhibit diminished activity (Tregear, G. W., et al., Endocrinology 93:1349–1353 (1973); Goltzman, D., et al., J. Biol. Chem. 250:3199–3203 (1975); Horiuchi, N., et al., Science 220:1053–1055 (1983) and Gardella, T. J., et al., J. Biol. Chem. 266:13141–13146 (1991)).
Several short NH2-terminal PTH or PTHrP peptides have been investigated previously, but no activity was detected. For example, bPTH(1–12) was inactive in adenylyl cyclase assays performed in rat renal membranes (Rosenblatt, M., “Parathyroid Hormone: Chemistry and Structure-Activity Relations,” in Pathobiology Annual, loachim, H. L., ed., Raven Press, New York (1981), pp. 53–84) and PTHrP(1–16) was inactive in AC assays performed in Chinese hamster ovary (CHO) cells expressing the cloned rat PTH-1 receptor (Azurani, A., et al., J. Biol. Chem. 271:14931–14936 (1996)). It has been known that residues in the 15–34 domain of PTH contribute importantly to receptor binding affinity, as the PTH(15–34) fragment binds weakly to the receptor, but this peptide does not activate AC (Naussbaum, S. R., et al., J. Biol. Chem. 255:10183–10187 (1980) and Gardella, T. J., et al., Endocrinology 132:2024–2030 (1993)).